As one means for treating malignant tumors, there is thermotherapy using microwaves or high-frequency electric currents as an energy source. When performing the thermotherapy, it is necessary to make sure that the temperature of an affected part reaches the target temperature. As a temperature measuring technique, there is a thermal camera using infrared rays. However, this is mainly used only for measuring a surface temperature and cannot measure a temperature in a living body through which no infrared rays can transmit. Therefore, to measure the temperature of an affected part, it is considered that a temperature probe such as a thermistor or a thermocouple is invasively inserted into a body. However, in such method, there are a problem of paining patients and a hygienic problem of causing infectious disease.
To solve such problems, a technique which can wirelessly measure the temperature of a portion to be measured from a position away from the portion to be measured has been considered and disclosed. For instance, Patent Document 1 discloses a temperature measuring method in which a temperature measuring element having a permanent magnet and a plurality of temperature-sensitive magnetic substances covering the periphery of the permanent magnet and having different Curie points is arranged in a portion to be measured, a leakage magnetic flux leaked from the temperature measuring element depending on temperature is detected by a magnetic sensor arranged in the position away from the temperature measuring element, and the temperature of the portion to be measured is measured according to the output.    Patent Document 1: Japanese Patent No. 3333875